The natural human heart provides the body with a pulsatile flow of blood corresponding to the filling and emptying (beating) of the various chambers of the heart. The instantaneous blood flow rate varies in a complex cyclical manner from near zero to some maximum rate, with the overall blood flow rate being a time weighted average.
The cardiopulmonary bypass circuits of heart-lung machines used in open-heart surgery typically utilize centrifugal or positive displacement (i.e. roller type) pumps to provide the motive power for circulation of the blood. These pumps provide an essentially constant flow rate of blood through the circuit at all times, the instantaneous rate and the average rate being nearly identical.
The basic roller pump consists of two rollers, 180 degrees apart, that rotate in a circle through a half circular raceway. A length of flexible tubing between ¼ and ⅝ inch inner diameter is placed between the rollers and the raceway. The rollers rotating in a circular movement compress the tubing against the raceway, squeezing the blood ahead of the rollers. The rollers are set to almost completely occlude the tubing, and operate essentially as a positive displacement pump, each passage of a roller through the raceway pumping the entire volume of the fluid contained in the tubing segment between the rollers. As a positive displacement pump, high positive pressures can be generated at the pump outlet and high suction (negative) pressures can be generated at the pump inlet. Roller pumps are typically driven by a constant speed motor which draws blood at a substantially constant rate.
Medical studies have suggested that pulsatile flow, being more physiologically correct than constant flow, may have a beneficial impact on the efficacy of the extracorporeal perfusion. This can result in improved patient outcomes following cardiac bypass surgery. Pulsatile flow is important for cerebral oxygenation and autoregulation, and for other tissue perfusion and capillary blood flow. Cerebral hypoperfusion is a known problem in cardiopulmonary bypass. Neonates require a pulsatile flow. Native pulsatile flow stimulates the endothelial cells that line normal blood vessels, causing them to elongate and secret local factors (endothelium-derived relaxing factor [nitric oxide] and prostaglandin I2 [PGI2, or prostacyclin]) into the vessel wall (intramural release) and into the blood stream (intraluminal release). These factors maintain vascular tone (vessel relaxation), inhibit clot formation on the vessel inner surface (platelet adhesion and aggregation), inhibit monocyte adherence and chemotaxis, and inhibit smooth muscle cell migration and proliferation. There are other effects. U.S. Pat. No. 5,643,172 associates failure to provide a pulsatile flow with high incidence of renal dysfunction during ECMO followed by recovery after the return to pulsatile flow.
Various ways have been proposed to mimic in a heart-lung machine the natural pulsatile flow of the heart, but none of them have so far been satisfactory. The simplest way of providing a pulsed flow is to cyclically clamp and unclamp the inlet or outlet line of the heart-lung machine's arterial pump. Clamping the pump inlet is not desirable since it can create very high suction pressures in the inlet which can damage the red blood cells, or in some cases even cause cavitation which can potentially release gas bubbles into the blood stream. Further, during the low flow or rest periods, the pump rotors spin on a stagnant volume of fluid, which may result in mechanical trauma to the blood cells. Clamping the pump outlet is not desirable in a centrifugal pump due to this mechanical trauma. Clamping the pump outlet is not desirable in a positive displacement pump since the rapid buildup of pressure in the lines can rupture the connections or tubing, potentially resulting in a catastrophic event.
A more acceptable way of creating pulsatile flow is to vary the speed of the pump in a cyclical manner. This is easily accomplished electronically by the pump controller. However, the inertia of the spinning elements of the pump tends to render the resulting waveform more sinusoidal than the natural heartbeat waveform and forces the wave period to be longer than the natural period. In addition, the components of the bypass circuit downstream of the pump, such as the oxygenator and arterial filter, also damp the pulses due to their volumetric holdup.
A pump in the prior art which, unlike the roller pump and the centrifugal pump, is provided with regulating devices to control the available pumping volume so the output is controlled as a function of inlet pressure, was invented by A. Sausse, described in U.S. Pat. No. 3,784,323, incorporated herein by reference. This pump, originally designed for use in hemodialysis, was commercialized for a period of time by Rhone-Poulenc, S.A., as the RP.01 through RP.06 series of pumps.
The Sausse (Rhone-Poulenc) pump stretches a distensible silicon tubing of an ovoid or elliptical cross section and shape memory compliance longitudinally around pin rollers mounted 120 degrees apart on a rotating wheel, the tubing being held in place below the wheel by connectors retained in a notched fixed base. This tubing, herein called a “header” tubing, is not compressed against a raceway (as for a roller pump), but is held in tension across the rollers, restricting the lumen of the header tubing across the rollers. This segments the header tubing into portions defined by leading and trailing adjacent rollers. The rotation of the wheel moves fluid captured between adjacent rollers in the direction of the rotation. The material and thickness of the wall of the header tubing are selected so the tubing between the rollers will expand or collapse as a function of pump inlet pressure (available venous return). Collapse of the tube will restrict the flow rate of the liquid as a function of the pump inlet pressure. If the venous supply decreases and inlet pressure drops, flow rate will lessen even though the pump speed is unchanged, and the inlet line will remain filled. Consequently, no dangerously low negative pressures can occur, unlike what is possible with roller and centrifugal pumps. When outflow obstruction occurs, the liquid blocked from flowing forward can back flow, so the pump feeds nothing forward to over pressurize and burst the return line. Instead, the back flow accumulates in the stretched header tubing, which distends or expands to accommodate the additional volume. When the obstruction is released, blood flows downstream propelled by the increased stroke volume of the distended header tubing. The header tubing stretched over the rollers therefore functions as a built-in capacitance reservoir, eliminating the need for the reservoirs that are required for roller and centrifugal pumps.
The flow rate of the Sausse type pump may be considered as substitute cardiac output and pump suction volume as diverted venous return. The compliance of the header tube allows its volume to increase under the action of the suction pressure. The volume is evacuated in the form of a bolus, and its evacuation causes the tube to regain a flat shape capable for being refilled. This compliance provides a level of security that is similar to that of a reservoir.
Lastly, a reciprocating type pump such as a diaphragm or bladder pump can be employed to create pulses in the flow. These pumps tend to be more mechanically complex than the roller or centrifugal types and do not lend themselves to either easy cleaning, sanitation, and sterilization for reuse, or low cost manufacture for one-time disposable use. Increased blood trauma is experienced in these pumps due to the multiple check valves in the flow path and stagnant areas due to less than perfect chamber filling and ejection. Lastly, as mentioned above, downstream components still damp the pulses and thus reduce the beneficial effects of the reciprocating pump.
The detailed rationale for pulsatile flow is described in a book co-authored by Linda B. Mongero and Kames R. Beck entitled “On bypass: advanced perfusion techniques”—published by Humana Press in 2008. Pages 102-114 deal with the issue of pulsatile flow during bypass and are incorporated herein in their entirety by reference.
Despite many attempts to deliver pulsatile flow, no blood pump adapted for this purpose is routinely used today. The need still exists for a simple peristaltic pump capable of delivering pulsatile flow during an open heart surgery.